a. Field of the Invention
The invention relates to a contact arrangement for vacuum switches with an axial magnetic field, having contact pieces which contain contact bodies as a base material. The contact bodies are soldered to contact carriers and, for the reduction of eddy currents, have radial areas of less electrical conductivity than the base material.
b. Description of the Prior Art
With the increasing spread of the vacuum switching in the medium--voltage range in devices, the control of higher cutoff currents is required. High currents even exceeding 40 kA must be shut off safely, while at the same time maintaining or even reducing the outside dimensions of the switching mechanism. Many special contact geometries have been suggested which conduct the switching current in an axial or longitudinal direction in the vicinity of the contact bodies. This generates between the contact pieces, during the switching operation, an axial magnetic field which brings about a switching arc distributed uniformly over the entire contact area.
In such contact configurations with axial magnetic field the following problem generally occurs: Due to the change in the current flowing across the contacts, eddy currents are induced in closed, ringshaped contact carriers in washer-shaped contact carriers or in contact rings. Such eddy currents generate secondary magetic fields which weaken the amplitude of the axial magnetic field and cause a phase shift relative to the current flowing through the switch. The effect of the phase shift of the axial magnetic field, however, is that during and after the current zero crossing a considerable residual axial magnetic field remains. This magetic field prevents the rapid discharge of the charge carriers within the contact gap and promotes an undesired reignition of the arc.
Various solutions for the prevention of eddy currents in vacuum switches with axial magnetic field have been proposed. For example, in DE-A No. 2 443 141 a contact arrangement is described with four hookshaped conductors extending radially and axially to generate the axial magnetic field and in which a diskshaped contact body is provided which is radially slotted for the prevention of eddy currents. DE-A No. 3 231 593 discloses a contact design of axial conductors which are formed by making several slots in the same direction in a cup contact to which a contact disk with radial slots is soldered. Moreover, in EP-A No. 0 055 088 conductors are illustrated shaped to generate the magnetic field, the current flowing repeatedly in the arc plane and the contact body being divided into several parts by wide slots.
The neccessity of radial slots in the contact body in contact arrangements for vacuum switches with axial magnetic field is also known in EP-A No. 0155 584 and EP-A No. 0 052 371. As an alternative thereto the use of suitable current barriers of a high-resistance material such as stainless steel and ceramic is also know. According to EP-A No. 0 052 371, such barriers can be inserted into the contact pieces with the actual contact body of a CuFe or CuCo alloy and the contact carrier underneath having the same geometry and consists of a CuPb or CuBi alloy, possible only at great cost.
In the technical literature (e.g., "IEEE Transactions on Power Apparatus and Systems" (1980), pages 2079 to 2085) conditions are described in detail under which slots are needed in the contact disk exposed to the arc for eddy current prevention.
Thus, in the present state of the art, the necessity of the slots as a suitable means of eddy current suppression is assumed. A disadvantage of this practice is that because of the broad, radial slots in the contact surface facing the arc, there are preferably formed at their edges cathode bases which can lead to reignitions due to thermal overheating. The same applies when the current barriers replacing the slots lead to macroscopic inhomogeneities on the switching surface of the contacts. Moreover, in this practice, the disks can be slotted only to about one third of the contact disk diameter to retain the stability of the contact body. Because of the lack of slots in the central area of the contact pieces, the eddy currents flowing there remain fully effective.